In these days of facile cloning and rapid DNA sequencing, it is not uncommon for investigators to find themselves with a DNA sequence that may or may not code for a known gene product. The sequence is 'open' when read in an appropriate frame, which is to say that there is a long run of amino acid codons before the appearance of a terminator codon. How can we find out if this 'unidentified reading frame' (URF) really codes for a genuine protein, and how can we identify it if it exists?